In high-frequency circuit radio communication devices such as cellular phones, laminated dielectric filters have been used as high-frequency circuit filters such as top filters, interstage filters for transmission, local filters, and interstage filters for reception. In the production of each laminated dielectric filter, plural molded bodies, of which a dielectric body is made, are formed with a ceramic powder, and a predetermined conductive paste is applied to each molded body to form a predetermined electrode pattern on the molded body. Then the molded bodies are laminated each other to give a laminate, and by firing the laminate, the conductive paste layer and the molded bodies are fired together, whereby the laminate is densified.
In the above step, the electrode is generally formed of a low-melting metallic conductor such as a silver-based, copper-based, or nickel-based conductor. Since the melting point of such conductor is below 1100° C., for example, and sometimes as low as about 930° C., it is necessary to sinter the dielectric body at a firing temperature lower than the melting point of the low-melting metal of which the electrode is formed.
To reduce stray capacitance, delay time and high-frequency losses of a built-in resonator and a capacitor, it is desired that the dielectric constant ∈r of low-temperature fired porcelain be lowered and the quality factor Q be increased. The present applicant has disclosed, in Japanese Patent Application Laid-Open No. 2000-211969A, a low-temperature fired porcelain having an optimum firing temperature of below 1000° C., a dielectric constant ∈r of below 10, and a quality factor Q of above 2500. This porcelain is mainly made of silica, alumina, and barium oxide, in which high-strength celsian phases are precipitated.
Furthermore, in Japanese Patent Application Laid-Open No. 2003-12370A, by precipitating sanbornite, celsian, and cristobalite phases, the firing shrinkage of a porcelain during firing is controlled and crack occurrence within the porcelain is prevented.
Also, Japanese Patent Application Laid-Open No. 2003-40668A discloses a low-temperature fired porcelain in which garnite, spinel, needle-shaped celsian crystalline phases and the like are precipitated and which has high strength, high thermal conductivity, and high Young's modulus.
However, as electronic parts are increased in frequency and decreased in size, there is an increasing demand to further enhance the fracture toughness of ceramic porcelains. In such ceramic porcelains as well, there is still an upper limit to the enhancement of the fracture toughness thereof, and thus there is a growing demand to implement the further enhancement of the fracture toughness thereof while maintaining other characteristics.